Hydrazide-containing thermoplastic polymers exhibiting nonlinear optical response

ABSTRACT

In one embodiment this invention provides a nonlinear optical medium which is a transparent film of a thermoplastic polymer composed of recurring monomeric units corresponding to the formula: ##STR1##

This invention was made with Government support under Contract No.F49620-85-0047 awarded by the Department of Defense. The FederalGovernment has certain rights in this invention.

This application is a division of application Ser. No. 093,680, filedSept. 8, 1987, now U.S. Pat. No. 4,818,802.

BACKGROUND OF THE INVENTION

It is known that organic and polymeric materials with large delocalizedπ-electron systems can exhibit nonlinear optical response, which in manycases is a much larger response than by inorganic substrates.

In addition, the properties of organic and polymeric materials can bevaried to optimize other desirable properties, such as mechanical andthermoxidative stability and high laser damage threshold, withpreservation of the electronic interactions responsible for nonlinearoptical effects.

Thin films of organic or polymeric materials with large second-ordernonlinearities in combination with silicon-based electronic circuitryhave potential as systems for laser modulation and deflection,information control in optical circuitry, and the like.

Other novel processes occurring through third-order nonlinearity such asdegenerate four-wave mixing, whereby real-time processing of opticalfields occurs, have potential utility in such diverse fields as opticalcommunications and integrated circuit fabrication.

Nonlinear optical properties of organic and polymeric materials was thesubject of a symposium sponsored by the ACS division of PolymerChemistry at the 18th meeting of the American Chemical Society,September 1982. Papers presented at the meeting are published in ACSSymposium Series 233, American Chemical Society, Washington, D.C. 1983.

The above-recited publications are incorporated herein by reference.

Of general interest with respect to the present invention is prior artrelating to thermoplastic polymers containing recurring pendant amide orimide groups, such as that described in U.S. Pat. Nos. 2,977,334;3,157,595; 3,684,777; 3,714,045; 4,121,026; 4,169,924; and 4,246,374.

Of particular interest with respect to the present invention arereferences which describe hydrazide compounds and hydrazide-containingpolymers, such as U.S. Pat. Nos. 2,763,539; 3,395,197; 3,840,600;4,083,835; and 4,171,413.

There is continuing research effort to develop new nonlinear opticalorganic systems for prospective novel phenomena and devices adapted forlaser frequency conversion, information control in optical circuitry,light valves and optical switches. The potential utility of organicmaterials with large second-order and third-order nonlinearities forvery high frequency application contrasts with the bandwidth limitationsof conventional inorganic electrooptic materials.

Accordingly, it is an object of this invention to provide organiccompositions which are characterized by a delocalized conjugatedx-electron system which can exhibit nonlinear optical response.

It is another object of this invention to provide novel thermoplasticpolymers which are characterized by recurring pendant hydrazidestructures which exhibit nonlinear optical response.

It is a further object of this invention to provide high performancenonlinear optical media and devices.

Other objects and advantages of the present invention shall becomeapparent from the accompanying description and examples.

The subject matter of the present patent application is related to thatdisclosed in copending patent application Ser. No. 854,274, filed Apr.21, 1986.

DESCRIPTION OF THE INVENTION

One or more objects of the present invention are accomplished by theprovision of a thermoplastic polymer which is characterized by arecurring monomeric unit corresponding to the formula: ##STR2## where Ris a substituent selected from hydrogen and C₁ -C₄ alkyl groups; and mis an integer of at least 3.

Illustrative of the R substituent in the above formula are C₁ -C₄ alkylgroups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl and2-butyl.

The weight average molecular weight of the thermoplastic polymer canvary in the range between about 500-500,000, and typically will be inthe range between about 5000-200,000.

The thermoplastic polymer can contain polymerized comonomeric units inaddition to the recurring acrylate monomeric unit represented in theformula above. It is preferred that the recurring acrylate monomericunits of the formula comprise at least about 20 weight percent of thetotal polymer weight.

Suitable comonomeric units include polymerized divalent residues ofethylene; vinyl halides such as vinyl chloride, vinylidene chloride, andvinyl fluoride; arylethylenes such as styrene, vinylnaphthalene andvinylcarbazole; acrylamides such as acrylamide, methacrylamide,N,N-dialkylacrylamide and N,N-dialkylmethacrylamide; acrylonitriles suchas acrylonitrile or methacrylonitrile, acrylates such as alkyl acrylate,alkyl methacrylate and alkyl alpha-phenylacrylate; and the like. Alsocontemplated are monomers with nonlinear optically responsive pendantside chains, such as 4-(6-methacryloxy)-4'-nitrobiphenyl.

In another embodiment this invention provides a transparent nonlinearoptical medium comprising a thermoplastic polymer which is characterizedby a recurring monomeric unit corresponding to the formula: ##STR3##where R is a substituent selected from hydrogen and C₁ -C₄ alkyl groups:and m is an integer of at least 3.

A present invention transparent nonlinear optical medium can be preparedby dissolving the thermoplastic polymer in a solvent such as toluene orN,N-dimethylformamide and spraying the solution on the surface of atransparent substrate such as optical glass to form a thin coating. Anonlinear optical medium also can be shaped by casting or molding a meltphase of the thermoplastic polymer to form a film, lens, prism, and thelike.

In another embodiment this invention provides a light switch or lightmodulator device with a polymeric nonlinear optical component comprisinga transparent solid medium of a polymer which is characterized by arecurring monomeric unit corresponding to the formula: ##STR4## where Ris a substituent selected from hydrogen and C₁ -C₄ alkyl groups; and mis an integer of at least 3.

In a further embodiment this invention provides a polymerizablecomposition corresponding to the formula: ##STR5## where R is asubstituent selected from hydrogen and C₁ -C₄ alkyl groups, and Z is--NO₂, --CN or --CF₃. The phenyl structure can contain two or more Zsubstituents.

The Z substituent on the phenyl structure in the above formulapreferably is in the para position.

The term "transparent" as employed herein refers to a liquid or solidmedium which is transparent or light transmitting with respect toincident fundamental light frequencies and harmonic light frequencies.

A nonlinear optical medium as defined above can be in the form of anoncentrosymmetric configuration of aligned polymer molecules, and themedium can exhibit a Miller's delta of at least about one squaremeter/coulomb. A noncentrosymmetric alignment of molecules can beinduced with an external field. When the polymer molecules are in arandom configuration, the medium exhibits third order opticalsusceptibility χ.sup.( 3) harmonic response.

The term "Miller's delta" as employed herein with respect to secondharmonic generation (SHG) is defined by Garito et al in Chapter 1,"Molecular Optics:Nonlinear Optical Properties Of Organic And PolymericCrystals"; ACS Symposium Series 233 (1983).

The quantity "delta" (δ) is defined by the equation:

    d.sub.ijk =ε.sub.o χ.sub.ii.sup.(1) χ.sub.jj.sup.(1) χ.sub.kk.sup.(1).sbsp.δ.sub.ijk

where terms such as χ_(ii).sup.(1) are the linear susceptibilitycomponents, and d_(ijk), the second harmonic coefficient, is definedthrough

    χ.sub.ijk (-2ω; ω)=2 d.sub.ijk (-2ω; ω,ω)

The Miller's delta (10⁻² m² /c at 1.06 μm) of various nonlinear opticalcrystalline substrates are illustrated by KDP (3.5), LiNbO₃ (7.5), GaAs(1.8) and 2-methyl-4-nitroaniline (160).

Preparation Of Thermoplastic Polymers

The following reaction diagram illustrates a general method ofsynthesizing a present invention thermoplastic polymer: ##STR6##

Another synthesis method is illustrated by the following reactiondiagram: ##STR7## The substituent Z in the above formulae is exemplifiedby a P-cyano or P-trifluoromethyl group.

Nonlinear Optical Properties

The fundamental concepts of nonlinear optics and their relationship tochemical structures can be expressed in terms of dipolar approximationwith respect to the polarization induced in an atom or molecule by anexternal field.

As summarized in the ACS Symposium Series 233 (1983) listed hereinabovein the Background Of The Invention section, the fundamental equation (1)below describes the change in dipole moment between the ground stateμ_(g) and and excited state μ_(e) expressed as a power series of theelectric field E which occurs upon interaction of such a field, as inthe electric component of electromagnetic radiation, with a singlemolecule. The coefficient α is the familiar linear polarizability, β andγ are the quadratic and cubic hyperpolarizabilities, respectively. Thecoefficients for these hyperpolarizabilities are tensor quantities andtherefore highly symmetry dependent. Odd order coefficients arenonvanishing for all structures on the molecular and unit cell level.The even order coefficients such as β are zero for those structureshaving a center of inversion symmetry on the molecular and/or unit celllevel.

Equation (2) is identical with (1) except that it describes amacroscopic polarization, such as that arising from an array ofmolecules in a polymer domain:

    Δμ=μ.sub.e -μ.sub.g =αE+βEE+γEEE+ . . . (1)

    P=P.sub.O +χ.sup.(1) E+χ.sup.(2) EE+χ.sup.(3) EEE+ . . . (2)

Light waves passing through an array of molecules can interact with themto produce new waves. This interacton may be interpreted as resultingfrom a modulation in refractive index or alternatively as a nonlinearityof the polarization. Such interaction occurs most efficiently whencertain phase matching conditions are met, requiring identicalpropagation speeds of the fundamental wave and the harmonic wave.

A present invention thermoplastic polymer medium typically is opticallytransparent and exhibits hyperpolarization tensor properties such asthird harmonic generation.

These theoretical considerations are elaborated by Garito et al inchapter 1 of the ACS Symposium Series 233 (1983); and by Lipscomb et alin J. Chem., Phys., 75, 1509 (1981), incorporated by reference. See alsoLalama et al, Phys Rev., A20, 1179 (1979); and Garito et al, Mol.,Cryst. and Liq. Cryst., 106, 219 (1984); incorporated by reference.

Field-induced Macroscopic Nonlinearity

The term "external field" as employed herein refers to an electric,magnetic or mechanical stress field which is applied to a substrate ofmobile organic molecules, to induce dipolar alignment of the moleculesparallel to the field.

The electronic origins of nonlinear optical effects in organicπ-electronic systems is reviewed by D. J. Williams in Angew. Chem., Int.Ed. Engl., 23, 690 (1984); incorporated herein by reference.

As described in the review article, a technique has been developed formeasuring B without necessitating the incorporation of the molecule intononcentrosymmetric crystal structures. In this technique, calledelectric-field induced second-harmonic generation (EFISH), a strong DCelectric field is applied to a liquid or a solution of the molecules ofinterest in order to remove the orientational averaging by statisticalalignment of molecular dipoles in the medium. The induced second-ordernonlinearity can then produce a signal at 2ω, from which β can beextracted.

A schematic diagram of experimental system for measurement of β by theEFISH technique is presented in the review article. As illustrated inthe published diagram, the 1.06 μm output of a Nd³⁺ :YAG laser is splitand directed into a sample and a reference cell. The sample cell istranslated by a stepped-motor-controlled stage across the beam. Thelaser pulse is synchronized with a high-voltage DC pulse to induceharmonic generation in the cell. The 0.53 μm radiation is separated fromthe 1.06 μm pump beam by filters and a monochromator, and the harmonicintensity is detected by a photomultiplier tube. The signal-to-noiseratio can be improved with a boxcar averager. The reference beam isdirected into a crystal such as quartz, whose second-order propertiesare well known, so that fluctuations in beam intensity can be readilycorrected in the output data. The value of the nonlinear coefficient isobtained from the ratio of the signals of the sample cell and areference material such as quartz or LiNbO₃ with known χ.sup.(2).

A present invention thermoplastic polymer is adapted to exhibit theexternal field-induced macroscopic nonlinearity required for secondorder harmonic generation.

The following examples are further illustrative of the presentinvention. The components and specific ingredients are presented asbeing typical, and various modifications can be derived in view of theforegoing disclosure within the scope of the invention.

EXAMPLE I

This Example illustrates a general procedure for the preparation of apendant hydrazide-containing thermoplastic polymer in accordance withthe present invention. ##STR8##

A copolymer of 20 molar percent of styrene and 80 molar percent ofacryloyl chloride is synthesized by solution polymerization of themonomers in N,N-dimethylformamide with 2.0 weight percentazodiisobutyronitrile catalyst at 75° C. for a period of six hours. Thecopolymer product has a weight average molecular weight of about 20,000.

The copolymer is reacted with a stoichiometric excess of2,4-dinitrophenylhydrazine in N,N-dimethylformamide solution at 80° C.to produce a thermoplastic polymer characterized by recurring acryloyl2,4-dinitrophenylhydrazide units.

EXAMPLE II

This Example illustrates the preparation of a pendanthydrazide-containing thermoplastic polymer in accordance with thepresent invention. ##STR9##

A 300 ml three-necked flask equipped with a mechanical stirrer, anaddition funnel, and nitrogen inlet and outlet, is charged with 15.3grams (100 mmoles) of 4-nitrophenylhydrazine and 60 ml ofdimethylacetamide. To the resulting mixture chilled in an ice-water bathis added 9.0 grams (100 mmoles) of acryloyl chloride over a period of 30minutes.

The resulting reaction mixture is stirred at 0°-5° C. for 3 hours, andpoured into 2 liters of ice-water to precipitate the product. The solidprecipitate is filtered, washed successively with water, 5% hydrochloricacid solution, water, 5% sodium bicarbonate, and water, and thenair-dried to provide 18.5 grams of crude product. The crude product isrecrystallized from 50:50 mixture of methanol and water to yield2-propenoic-2'-(p-nitrophenyl)hydrazide, m.p. 204° C.

The second harmonic susceptibility of the powder sample is 1×10⁻⁵watt/square centimeter at 532 nm when Nd:YAG pulsed laser at 1064 nm isemployed as a fundamental wave source.

Following the procedure of Example I, the2-propenoic-2'-(p-nitrophenyl)hydrazide is polymerized to form athermoplastic polymer characterized by recurring pendant hydrazidegroups which exhibit second order nonlinear optical susceptibility β.

The thermoplastic polymer has a weight average molecular weight of about10,000, and a melting point of 180° C.

The corresponding pendant 4-cyanophenylhydrazide or4-trifluoromethylphenylhydrazide derivatives can be prepared byutilizing 4-cyanophenylhydrazine or 4-trifluoromethylphenylhydrazine inplace of 4-nitrophenylhydrazine in the monomer preparation.

EXAMPLE III

This Example illustrates the preparation of a pendanthydrazide-containing thermoplastic polymer in accordance with thepresent invention. ##STR10##

Following the procedure of Example II,2-methyl-2-propenoic-2'-(p-nitrophenyl)hydrazide (m.p. 170° C.) isprepared by employing a dimethylacetamide solution containing 15.3 grams(100 mmoles) of 4-nitrophenylhydrazine and 10.45 grams (100 mmoles) ofmethacryloyl chloride. The second harmonic susceptibility of a powdersample is 0.3×10³¹ 5 watt/square centimeter at 532 nm when Nd:YAG pulsedlaser at 1064 nm is employed as a fundamental wave source.

Employing the polymerization conditions of Example I, except that thecatalyst is dicumyl peroxide and the temperature is 140° C.,2-methyl-2-propenoic-2'-(p-nitrophenyl)-hydrazide is polymerized to formpoly(methacryloyl 4-nitrophenylhydrazide) having a weight averagemolecular weight of about 20,000-25,000.

A N,N-dimethylacetamide solution of the thermoplastic polymer is sprayedon an optical glass surface. The operation is repeated to form atransparent multilayer coating on the glass surface.

The coating can exhibit a third order susceptibility χ.sup.(3) of about1×10⁻¹⁰ esu as measured at 1.91 μm excitation wavelength.

EXAMPLE IV

This Example illustrates the preparation of a thin substrate ofthermoplastic polymer with a macroscopic noncentrosymmetric molecularorientation in accordance with the present invention.

Poly(acryloyl 4-nitrophenylhydrazide) polymer as described in Example IIis compression molded to form a film of about 500 micron thickness.

The molding is accomplished in a 30 ton press (Wabash Metal Products,Inc. Model #30-1010-2TMX) with programmed heating and cooling, andadjustable pressure. The platen temperature is set at 200° C. Thepolymer in particulate form is placed between two Kapton (DuPontpolyimide) sheets and positioned between the two platens. The platensare closed and 6 tons pressure is applied for 2 minutes. The platens arethen cooled to 100° C. within thirty seconds, the pressure is released,and the film sample is retrieved from the press.

X-ray diffraction patterns from this film sample, record by using nickelfiltered CuK.sub.α radiation and flat plate photographic techniques,indicate a random orientation of polymer molecular axes.

Molecular alignment of the polymer molecule axes is achieved in thefollowing manner. The film sample is sandwiched between two Kapton filmsof 0.002 inch thickness which in turn are sandwiched between two metalplates of 0.25 inch thickness, each having a ground flat surface and arod attached to one side which serves as a contact for application ofvoltage in the alignment procedure. The sub-assembly is covered on topand bottom with a double layer of Kapton sheets of 0.002 inch thicknessand providing a 0.004 inch electrical insulating layer against eachplaten.

The whole assembly is placed between the platens of the press previouslyemployed for preparing the unoriented precursor film sample. The platensare preheated to 200° C., then closed and a pressure of 6 tons isapplied. Wires from a DC power supply are attached to the rods of theelectrode plates and a voltage of 700 V is applied for two hours whilemaintaining temperature and pressure.

The press is cooled rapidly to 100° C. while pressure and voltage aremaintained. At that temperature, the voltage is reduced to zero and thepressure released. The molecularly aligned film sample is retrieved fromthe mold, and X-ray diffraction patterns are recorded with nickelfiltered CuK.sub.α radiation and wide-angle photographic flat platetechniques. Orientation functions are determined utilizing a polar tableand a microdensitometer interfaced with a LeCray computer.

The data demonstrate that the molecular alignment process results in arotation of essentially all of the molecular axes of the polymermolecules into a direction parallel to that of the external field. Thistype of molecularly aligned polymer film is noncentrosymmetric and canfunction as a second-order harmonic-generating nonlinear optical mediumfor a high intensity light field to which the medium is optically clear,e.g., as the nonlinear optical component in a light switch or lightmodulator device, with a Miller's delta of at least about one squaremeter/coulomb.

What is claimed is:
 1. A polymerizable compound corresponding to theformula: ##STR11## where R is a substituent selected from hydrogen andC₁ -C₄ alkyl groups, and Z is p-cyano or p-trifluoromethyl.
 2. Acomposition in accordance with claim 1 wherein R is hydrogen.
 3. Acomposition in accordance with claim 1 wherein R is methyl.
 4. Acomposition in accordance with claim 1 wherein Z is p-cyano.
 5. Acomposition in accordance with claim 1 wherein Z is p-trifluoromethyl.